The preparation by a sol-gel process of dense, alumina-based ceramic abrasive grain is known. U.S. Pat. No. 4,314,827 describes a process of making an abrasive mineral employing chemical ceramic technology by gelling alumina monohydrate with a precursor of at least one modifying component followed by dehydration and firing. A preferred method of addition of the modifying component is in the form of a soluble salt such as magnesium nitrate. other disclosures in this area include U.S. Pat. No. 4,181,532, "Application of Sol-Gel Processes to Industrial Oxides", Jan. 13, 1968, Chemistry and Industry, and the following United States patents and application, all of which are assigned to the assignee of the present application:
M. A. Leitheiser et al., U.S. Pat. No. 4,518,397, entitled "Non-fused Aluminum Oxide-Based Abrasive Mineral";
A. P. Gerk, U.S. Pat. No. 4,574,003, entitled "Process for Improved Densification of Sol-Gel Produced Alumina-Based Ceramics", and
A. P. Gerk et al, Ser. No. 666,133, filed Oct. 30, 1984, entitled "Superior High Sodium and Calcium Abrasive and Process for its Production".
These references disclose techniques for making alumina based ceramic material useful as abrasive grains. The ceramic material which results from most, if not all, of these processes is generally characterized by having identifiable "domains" formed of collections of usually similarly oriented crystals of alpha alumina. These domains typically have average diameters on the order of 10 micrometers or larger, with the smallest average diameter being about 6 micrometers.
Other references disclose the enhanced densification of alumina sol-gels by alpha alumina seeding. For example, M. Kumagai and G. L. Messing spoke at the American Ceramic Society meeting on May 2, 1984 on this subject and later caused the publishing of a paper in November, 1984 in Communications of the American Ceramic Society entitled "Enhanced Densification of Boehmite Sol-Gels by .alpha.-Alumina Seeding". Other references on this subject include an article entitled, "Alpha Alumina Formation in Alum-Derived Gamma Alumina" by F. W. Dynys and J. W. Halloran Journal of the American Ceramic Society, Vol. 65, No. 9, p. 442-448, December, 1982, and several papers presented Feb. 25-Mar. 1, 1985 at the Second International Conference on Ultra Structure Processing of Ceramics, Glasses and Composites, including the following: R. Roy et al. of Pennsylvania State University "Nucleation and Epitaxial Growth in Di-Phasic Gels"; and G. Messing et al. of Pennsylvania State University "Transformation and Sintering of Seeded Boehmite Gels". Cottringer et al., U.S. Pat. No. 4,623,364, discloses seeding alumina gels by introduction of seed material as by wet vibratory milling of the material with alumina media, or by the direct addition of very fine alpha alumina particles.